The proposed existence of adult, marrow-derived stem cells that retain the ability to generate various tissues, is an appealing concept with considerable therapeutic potential. With few exceptions the data used to support this stem cell plasticity concept come from murine studies in which immune cytochemistry and molecular probes are used to identify rare donor cells in fixed tissue. Whether these observations are valid, and if valid, whether they pertain to larger longer lived animals remains controversial. To contribute to the resolution of this uncertainty we propose to investigate the contribution of marrow stem cells to skin, liver, and marrow stroma. We hypothesize that if "plastic stem cells" reside in marrow they will contribute to nonhematopoietic tissue regeneration as needed. To test this we will investigate the effect of tissue specific proliferative demand on the detection of donor cells in canine stem cell recipients. All dogs used in these studies will be long term stable chimeras generated under the auspices of DK51 417. Briefly, under general anesthesia the chimeric dogs will undergo liver lobectomy, split skin harvest, and marrow aspiration. The harvested tissue will be assessed for % donor cells, the animals will recover, and after 3-5 months the regenerated tissue will also be assessed for % donor cells. Importantly tissue samples will be cultured ex vivo to expand cell populations of interest thereby providing relatively large numbers of morphologically distinct and cytochemically defined cells. Polymerase chain reaction (pcr) amplification of donor versus host DNA will then be used to determine the donor contribution to these defined cell populations. These strategies should improve the signal to noise ratio and make the detection of donor derived cells more reliable. The information gained should help define whether, and under what circumstances, marrow derived stem cells contribute to other tissues following allogeneic transplantation.